1. Field of the Invention
The present invention relates to a frame synchronous circuit for a dedicated short-range communication (DSRC) system, and, in particular, for a DSRC system which accepts signal waves modulated with either an amplitude-shift keying (ASK) or quadrature phase shift keying (QPSK) scheme.
2. Description of the Background Art
In recent years, a roadside-vehicle communication system employing the DSRC system complying with a radio communication standard, such as the Standard ARIB STD-T75, has been developed, and by utilizing such a communication system, an ETC (Electronic Toll Collection) service has started. The Standard ARIB STD-T75, ver. 1.3, was issued on Nov. 30, 2005 for further enhancing the service and increasing the communication speed, in which two modulation schemes are mixed such that π/4 shift QPSK scheme is added to the modulation scheme used in the conventional ETC system, i.e. ASK scheme. Accordingly, as an ETC system employing the DSRC system uses the communication schemes of ASK and QPSK, an on-board unit or station needs to be capable of using both schemes in communication with roadside units or stations.
In the radio communication systems including PHSs (Personal Handyphone System) and cellular phone systems as well as the DSRC systems and the ETC systems employing the DSRC system, a data unit called frame is used. For example, the PHS system uses frames which are composed of 640-bit data. A frame transmitted from a transmitter end includes at its head a synchronous word, e.g. a unique word (UW) in the DSRC system. The on-board unit detects the synchronous word to establish synchronization with roadside units. In such systems, some of the roadside units transmit the frame data in the ASK and QPSK schemes in an alternate manner, i.e. transmit them in the time-sharing manner in which the UW data are different between the ASK frame and the QPSK frame.
However, while the on-board unit is in synchronization, there is a possibility that data identical with the UW data is included in a data stream being received. Therefore, in order to prevent resynchronization from being erroneously established when a data pattern similar to the synchronous word pattern is detected, during receiving a frame from which a synchronous word for establishing synchronization is detected, at a timing other than the timing when a synchronous word is to be transmitted in the beginning of the frame subsequent thereto, a technique called perch channel is applied.
Furthermore, due to a bad air condition, data including a synchronous word sent from a base station may be regarded erroneous as a result of a cyclic redundancy check (CRC). The on-board unit can therefore use an error-free technique for requesting the base station to retransmit the data. On this occasion, the on-board unit uses a synchronous protection technique for avoiding loss of synchronization even if the unit fails to receive frames continuously.
The perch channel technique and synchronization protection technique are used as known art for the communication system such as the ETC system which employs the ASK modulation scheme dedicatedly as its modulation scheme in which the transfer speed and the position of the synchronous word are uniquely determined.
Japanese patent laid-open publication No. 162406/1995 discloses a frame synchronous detection and protection circuit which employs a synchronous scheme that opens a window at a timing predicted to receive a frame synchronous signal in response to frames, which are different in length, transmitted in a time-sharing manner.
Another Japanese patent laid-open publication No. 2004-147016 discloses a receiver which can demodulate both ASK and QPSK modulation waves.
On-board equipment in the DSRC system, such as an ETC on-board unit, includes a conventional frame synchronous circuit. The on-board unit generally operates on receiving FCMC (Frame Control Message Channel) data from a roadside unit in communication with and analyzing the received data to decode commands from the roadside unit. However, such an on-board unit has a disadvantage that when the unit performs a debug or test operation, the unit has to receive the FCMC data and set the operation to be performed only for sending data.
In addition, when both of the ASK and QPSK modulation schemes are used together and the ASK frames and QPSK frames are transmitted in the time-sharing manner, the conventional frame synchronous circuit can detect UW words included in signals in the modulation scheme firstly synchronized therewith, but not respond to signal waves using the other scheme until the current synchronization is cancelled.
Furthermore, when frames different in number of time slots are transmitted in the time-sharing fashion and a synchronous word fails to be detected in the second frame, use of the perch channel technique results in failing to detect a synchronous word in the third frame, thus failing to appropriately proceed to the time-sharing process, resulting in inefficient communications.
In the conventional frame synchronous circuit, the above disadvantages are difficult to overcome since synchronization in a radio communication and perch channel control during maintaining synchronization would be complicated to the extent that the design and manufacturing costs are expensive.
The conventional circuit also cannot cancel the synchronization until the completion of a frame under reception. When the on-board unit goes out of the service area of the roadside unit in synchronization therewith, it often cannot complete the frame reception due to the decrease in the intensity of the radio waves. Moreover, when the on-board unit is in synchronization with one roadside unit, a perch window is not open at a timing interval with another roadside unit, so that it could not detect a UW word contained in data transmitted from the other roadside unit.
In some cases, even if the on-board unit incorporating the conventional frame synchronous circuit receives FCMC data containing a UW word from and successfully synchronizes with the roadside unit, the circuit may not operate properly due to possible discrepancies in the FCMC data. Nevertheless, the synchronization is kept, and thereby the on-board unit cannot establish synchronization with another roadside unit.